Geomagnetism & Paleomagnetism
Geomagnetism & Paleomagnetism
Earth's magnetic field is generated by convection in the liquid outer core. Paleomagnetic records preserved in rocks document reversals of the field and the motion of tectonic plates — key evidence for plate tectonics.
Definition
Geodynamo: the outer core conducts molten iron. Differential rotation plus convection drives the magnetohydrodynamic (MHD) dynamo. The magnetic Reynolds number \(R_m = UL/\eta \gg 1\) (\(\eta\) = magnetic diffusivity) enables self-sustained field generation.
Key Result
The Glatzmaier-Roberts MHD simulation (1995) reproduced spontaneous polarity reversals over simulated thousand-year timescales, confirming that reversals arise naturally from the nonlinear geodynamo equations.
Example 1
Magnetic anomaly stripes on the ocean floor record field polarity during seafloor spreading. The Brunhes-Matuyama reversal 780,000 years ago is recorded as a stripe boundary parallel to the mid-Atlantic ridge.
Example 2
Virtual geomagnetic poles (VGPs) track apparent polar wander. When sampled from multiple continents, pre-Triassic VGPs converge only if the continents are moved to their paleopositions — direct evidence of continental drift.
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Practice
- How does the magnetohydrodynamic dynamo work?
- What is secular variation of the geomagnetic field?
- Explain how paleomagnetism provided evidence for seafloor spreading.
- Why is the geomagnetic field important for life on Earth?
Show Answer Key
1. Convective motion of electrically conducting liquid iron in the outer core generates and sustains the magnetic field through a self-exciting dynamo process. Fluid motion → electric currents → magnetic field → Lorentz force → modified fluid motion (positive feedback loop). Requires sufficient convective vigor (magnetic Reynolds number $\text{Re}_m > 10$).
2. Slow changes in the main field's strength, direction, and morphology over years to centuries. Caused by changes in outer core convection patterns. The dipole is currently weakening at ~5%/century, and the magnetic poles are drifting (North Magnetic Pole moving toward Siberia at ~50 km/yr).
3. Rocks record the ambient magnetic field direction when they cool below the Curie temperature (thermoremanent magnetization). Symmetric magnetic stripe patterns on the seafloor, parallel to mid-ocean ridges, show alternating normal/reversed polarity — matching the geomagnetic reversal timescale. This proved that new crust forms at ridges and spreads laterally (Vine-Matthews-Morley hypothesis, 1963).
4. The magnetosphere deflects solar wind and cosmic radiation. Without it, the solar wind would strip away the atmosphere (as happened on Mars after its dynamo stopped ~4 Gya). It also prevents direct exposure to energetic charged particles that would damage DNA and destroy the ozone layer.